Trans-Atlantic Geotraverse Hydrothermal Field Research Articles

Diversity, spatial distribution and evolution of inactive and weakly active hydrothermal deposits in the TAG hydrothermal field

Introduction: Although, there is an increasing focus on inactive or extinct seafloor massive sulfide (SMS) deposits driven by the possibility of marine mining, only few studies have been devoted to them so far. The Trans-Atlantic Geotraverse (TAG) hydrothermal field is probably one of the best-studied hydrothermal systems even if the relict SMS deposits known since the mid-1980s have not been thoroughly explored.Objectives: The main objective of this study was to describe the characteristics of these so-called inactive sites.Methods: During four different expeditions, we acquired high-resolution acoustic data and performed numerous human occupied vehicle (HOV) dive operations including extensive rock sampling and in-situ temperature measurements.Results and Discussion: We discovered thirteen new hydrothermal mounds including six large (i.e. > 5,000 m2) deposits making the TAG hydrothermal field one of the largest accumulation of hydrothermal materials (21.1 Mt) known on the seafloor. However, copper and zinc grades of the largest SMS deposits remain low (i.e. < 1.4 wt%) even compared to on-land volcanogenic massive sulfide deposits. Additionally, eight areas of diffuse hydrothermal fluid flow were identified challenging the presumed inactivity of these SMS deposits and, for the first time, emphasizing the importance of low temperature (LT) hydrothermal activity in whole the TAG field. Inactive and weakly active SMS deposits exhibit a large diversity of surface mineralization (e.g. sulfides, Fe-Mn mineralization, jasper) illustrating complexity of hydrothermal activities but also different ageing history. Several mounds no longer have visible sulfide chimneys and are covered by a widespread layer of manganese and iron oxyhydroxides attesting the longevity of diffuse fluid flow at specific locations even long after last high-temperature (HT) hydrothermal activity has ceased. This contrasts with SMS deposits that are devoid of extensive LT precipitates but characterized by standing or topped sulfide chimney indicating a relatively abrupt cessation of HT hydrothermal activity.Conclusion: Together these results allow us to propose evolution models to explain the diversity of active, weakly active and inactive SMS deposits in the TAG hydrothermal field.

Dynamic event-triggered observer-based control for autonomous underwater vehicles in the Trans-Atlantic Geotraverse hydrothermal field using rotation matrices

The issue of dynamic event-triggered trajectory tracking control of autonomous underwater vehicles (AUVs) in the Trans-Atlantic Geotraverse (TAG) active mound is addressed. Concretely, a novel adaptive event-triggered disturbance observer along with auxiliary functions is constructed to handle with actuator faults and ocean current in the TAG active mound. By using rotation matrices, a new tracking error system involving position and attitude is developed such that potential singularities caused by the large pitch will be sufficiently avoided. By deep combination with model parameters, a sliding mode manifold based on additional velocity errors is established to promote robustness and convergence. Following the detailed analysis, sufficient conditions for the input-to-state practical stability in mean square are given to ensure the tracking performance. An adaptive dynamic event-trigger method is applied to reduce the communication frequency between the embedded PC module and actuators. Finally, the applications of an AUV tracking system in the near-bottom hydrothermal mound are carried out to verify the efficiency of the presented method.

Adaptive Disturbance-Observer-Based Continuous Sliding Mode Control for Small Autonomous Underwater Vehicles in the Trans-Atlantic Geotraverse Hydrothermal Field with Trajectory Modeling Based on the Path

Considering intense hydrothermal activities and rugged topography in a near-bottom environment of the trans-Atlantic geotraverse (TAG) hydrothermal mound, a small autonomous underwater vehicle (S-AUV) will suffer from time-varying disturbances, model uncertainties, actuator faults, and input saturations. To handle these issues, a fault-tolerant adaptive robust sliding mode control method is presented in this paper. Firstly, unknown disturbances, model uncertainties, and actuator faults of the S-AUV are synthesized into a lumped uncertain vector. Without requiring the upper bound and gradient of the uncertainties, a continuous adaptive finite-time extended state observer is designed to estimate the lumped uncertain vector. Then, an auxiliary dynamic system composed of continuous functions is introduced to deal with input saturations, thereby contributing to achieving fixed-time trajectory tracking control of S-AUVs. Based on a designed continuous fixed-time nonsingular fast sliding mode surface, the proposed continuous adaptive controller is chattering free. Simulated topography is built according to topographic data of the TAG mound, and a smooth trajectory model is constructed by cubic spline interpolation. Comprehensive simulations performed on an actual S-AUV model are given to validate the effectiveness and superiority of the presented algorithm.

Structural Control, Evolution, and Accumulation Rates of Massive Sulfides in the TAG Hydrothermal Field

AbstractThe Trans‐Atlantic Geotraverse (TAG) hydrothermal field on the Mid‐Atlantic Ridge is one of the best‐studied hydrothermal systems to date. However, high‐resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle (AUV) reveal new information about the distribution of active and inactive hydrothermal deposits, and their relation to structural features. The discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow‐spreading ridges. The interpretation of ship‐based and high‐resolution AUV‐based data sets allowed for the determination of the main tectonic stress regimes that have a first‐order control on the location and distribution of past and present hydrothermal activity. The data reveal the importance of cross‐cutting lineament populations and temporal variations in the prevalent stress regime. A dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material (~29 Mt). The accumulation rate of ~1,500 t/yr is comparable to those of other modern seafloor vent fields. However, our observations suggest that the TAG segment is different from many other slow‐spreading ridge segments in its tectonic complexity, which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation. The inactive and weakly active mounds contain almost 10 times the amount of material as the active high‐temperature mound, providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits.

Analysis of deep-ocean sediments from the TAG hydrothermal field (MAR, 26° N): application of short-wave infrared reflectance (SWIR) spectra for offshore geochemical exploration

PurposeThe cost-efficient methods of analysis, such as rapid short-wave infrared (SWIR) spectral analysis, have been applied for the efficient exploration of critical raw materials (CRM), including mineral components and rare earth elements (REE) from the deep-ocean sediments.MethodsGravity cored sediment samples were collected during an oceanographic mission to the Trans-Atlantic Geotraverse (TAG) hydrothermal field of the Mid-Atlantic Ridge (MAR, 26° N). SWIR reflectance spectra (dependent variable) of samples were mathematically tested against referent geochemical data (independent variable), obtained by conventional analysis (ICP/OES, ICP/MS), after applied full cross-validation multivariate partial least square regression (CVPLSR). Value of parameter-residual predictive deviation (RPD) was used for evaluation of CVPLSR modeling: RPD > 2.5 (satisfactory calibration model for the screening purposes) and RPD > 5.0 (model adequate for the quality control of the studied elements).ResultsThe CVPLSR modeling provided significant results for the determination of several mineral components: major elements (Fe and Si) had the values of RPD equal to 3.65 and 2.84, respectively, which indicated a viable potential for their routine analysis, whereas RPD for Ca was equal to 5.51, thus assuring its quality control by SWIR analysis, in sediment samples of the studied location. Among the REE, Ce (RPD = 2.55) and Er (RPD = 2.59) yielded the most satisfactory results.ConclusionsThe findings highlight the benefit of rapidly obtained empirical SWIR-reflectance data, which can be used for near-real-time exploration of geochemical deposits hosted in deep-ocean sediments.

Underwater Hyperspectral Imaging Using a Stationary Platform in the Trans-Atlantic Geotraverse Hydrothermal Field

Underwater hyperspectral imaging is a relatively new method for characterizing seafloor composition. To date, it has been deployed from moving underwater vehicles, such as remotely operated vehicles and autonomous underwater vehicles. While moving vehicles allow relatively rapid surveying of several 10–1000 m2, they are subjected to short-term variations in vehicle attitude that often compromise image acquisition and quality. In this study, we tested a stationary platform that was landed on the seabed and used an underwater hyperspectral imager (UHI) on a vertical swinging bracket. The imaged seafloor areas have dimensions of 2.3 m $\times $ 1 m and are characterized by very stable UHI data of high spatial resolution. The study area was the Trans-Atlantic Geotraverse hydrothermal field at the Mid-Atlantic Ridge (26° N) in water depths of 3530–3660 m. UHI data were acquired a 12 stations on an active and an inactive hydrothermal sulfide mound. Based on supervised classification, 24 spectrally different seafloor materials were detected, including hydrothermal and non-hydrothermal materials, and benthic fauna. The results show that the UHI data are able to spectrally distinguish different types of surface materials and benthic fauna in hydrothermal areas, and may therefore represent a promising tool for high-resolution seafloor exploration in potential future deep-sea mining areas.

The Trans-Atlantic Geotraverse hydrothermal field: A hydrothermal system on an active detachment fault

Over the last ten years, geophysical studies have revealed that the Trans-Atlantic Geotraverse (TAG) hydrothermal field (26°08′N on the Mid-Atlantic Ridge) is located on the hanging wall of an active detachment fault. This is particularly important in light of the recognition that detachment faulting accounts for crustal accretion/extension along a significant portion of the Mid-Atlantic Ridge, and that the majority of confirmed vent sites on this slow-spreading ridge are hosted on detachment faults. The TAG hydrothermal field is one of the largest sites of high-temperature hydrothermal activity and mineralization found to date on the seafloor, and is comprised of active and relict deposits in different stages of evolution. The episodic nature of hydrothermal activity over the last 140ka provides strong evidence that the complex shape and geological structure of the active detachment fault system exerts first order, but poorly understood, influences on the hydrothermal circulation patterns, fluid chemistry, and mineral deposition. While hydrothermal circulation extracts heat from a deep source region, the location of the source region at TAG is unknown. Hydrothermal upflow is likely focused along the relatively permeable detachment fault interface at depth, and then the high temperature fluids leave the low-angle portion of the detachment fault and rise vertically through the highly fissured hanging wall to the seafloor. The presence of abundant anhydrite in the cone on the summit of the TAG active mound and in veins in the crust beneath provides evidence for a fluid circulation system that entrains significant amounts of seawater into the shallow parts of the mound and stockwork. Given the importance of detachment faulting for crustal extension at slow spreading ridges, the fundamental question that still needs to be addressed is: How do detachment fault systems, and the structure at depth associated with these systems (e.g., presence of plutons and/or high permeability zones) influence the pattern of hydrothermal circulation, mineral deposition, and fluid chemistry, both in space and time, within slowly accreted ocean crust?

A zonal picture of the water column distribution of dissolved iron(II) during the U.S. GEOTRACES North Atlantic transect cruise (GEOTRACES GA03)

We report measurements of the transient species iron(II) in filtered water column samples collected during the U.S. GEOTRACES North Atlantic transect cruise (GEOTRACES GA03), which was comprised of legs from Lisbon to Cape Verde in October–November 2010, and from Woods Hole to Cape Verde in November–December 2011. Dissolved iron(II) (dFe(II)) was determined at sea in 0.2 µm filtered samples as soon as possible after collection and filtration, using flow injection analysis, with mean detection limits of 0.06nM and 0.01nM during the 2010 and 2011 cruise legs, respectively. Water column concentrations along the cruise transects were generally low (<0.2nM), with the exception of deep water samples collected over the Trans Atlantic Geotraverse hydrothermal field, in which dFe(II) was as high as 70nM and accounted for more than 80% of the dissolved iron pool in the near-field hydrothermal plume. Smaller local concentration maxima were observed near 1000m depth in the low-oxygen, iron-rich waters to the east of Cape Verde, where dFe(II) is correlated with apparent oxygen utilization, and in the upper water column at several stations in the Subtropical Gyre, where dFe(II) can account for greater than 50% of the dissolved iron pool in the lower euphotic zone. Elevated dFe(II) concentrations were also observed over much of the water column on the Bermuda platform, although the source of this enrichment remains uncertain, in the absence of data between Woods Hole and Bermuda.

The changing vision of marine minerals

Non-fuel marine minerals are reviewed from the perspective of resources and their value as active analogs that can advance understanding of types of ancient ore deposits that formed in marine settings. The theory of plate tectonics is the largest influence in expanding our vision of marine minerals and in developing our understanding of geologic controls of mineralization in space and time. Prior to the advent of plate tectonics, we viewed the ocean basins as passive sinks that served as containers for particulate and dissolved material eroded from land. This view adequately explained marine placer deposits (heavy minerals and gems), aggregates (sand and gravel), and precipitates (phosphorites and manganese nodules). Although numerous sites of placer mineral deposits are known on continental shelves worldwide, current activity pertains to diamond mining off southwestern Africa, tin mining off southeastern Asia, and intermittent gold mining off northwestern North America, which are all surpassed economically by worldwide recovery of marine sand and gravel, in turn dwarfed by offshore oil and gas. With the advent of plate tectonics, plate boundaries in ocean basins are recognized as active sources of mineralization in the form of hydrothermal massive sulfide deposits and proximal lower-temperature deposits hosted in oceanic crust (mafic at ocean ridges and felsic at volcanic island arcs), and of magmatic Ni–Cu sulfide, chromite and PGE deposits inferred to be present in the oceanic upper mantle–lower crust based on their occurrence in ophiolites. Some 300 sites of hydrothermal active and relict mineralization, most of them minor, are known at this early stage of seafloor exploration on ocean ridges, in fore-arc volcanoes, at back-arc spreading axes, and in arc rifts; deposits formed at spreading axes and transported off-axis by spreading are present in oceanic lithosphere but are virtually unknown. The TAG (Trans-Atlantic Geotraverse) hydrothermal field in the axial valley of the Mid-Atlantic Ridge (latitude 26° N) is considered to exemplify a major Volcanogenic Massive Sulfide (VMS) deposit forming at a spreading axis. The most prospective of these occurrences lie within the 200 nautical mile (370 km)-wide Exclusive Economic Zone (EEZ) of the nations of the volcanic island arcs of the western Pacific where metal content of massive sulfides (Ag, Au, Ba, Cu, Pb, Sb, Zn) exceeds that at ocean ridges. Plate tectonics early provided a framework for mineralization on the scale of global plate boundaries and is providing guidance to gradually converge on sites of mineralization through regional scales of plate reorganization, with the potential to elucidate the occurrence of individual deposits (e.g., Eocene Carlin-type gold deposits). Investigation of the spectrum of marine minerals as active analogs of types of ancient mineral deposits is contributing to this convergence. Consideration of questions posed by Brian Skinner (1997) of what we do and do not know about ancient hydrothermal mineral deposits demonstrates the ongoing advances in understanding driven by investigation of marine minerals.

Crustal structure of the Trans‐Atlantic Geotraverse (TAG) segment (Mid‐Atlantic Ridge, 26°10′N): Implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges

New seismic refraction data reveal that hydrothermal circulation at the Trans‐Atlantic Geotraverse (TAG) hydrothermal field on the Mid‐Atlantic Ridge at 26°10′N is not driven by energy extracted from shallow or mid‐crustal magmatic intrusions. Our results show that the TAG hydrothermal field is underlain by rocks with high seismic velocities typical of lower crustal gabbros and partially serpentinized peridotites at depth as shallow as 1 km, and we find no evidence for low seismic velocities associated with mid‐crustal magma chambers. Our tomographic images support the hypothesis of Tivey et al. (2003) that the TAG field is located on the hanging wall of a detachment fault, and constrain the complex, dome‐shaped subsurface geometry of the fault system. Modeling of our seismic velocity profiles indicates that the porosity of the detachment footwall increases after rotation during exhumation, which may enhance footwall cooling. However, heat extracted from the footwall is insufficient for sustaining long‐term, high‐temperature, hydrothermal circulation at TAG. These constraints indicate that the primary heat source for the TAG hydrothermal system must be a deep magma reservoir at or below the base of the crust.

Native nickel in the TAG hydrothermal field sediments (Mid‐Atlantic Ridge, 26°N): Space trotter, guest from mantle, or a widespread mineral, connected with serpentinization?

Tiny metallic particles have been found in the sediments of the Trans‐Atlantic Geotraverse (TAG) hydrothermal field, Mid‐Atlantic Ridge (MAR). The metallic grains are almost pure Ni and have compositional and structural characteristics similar to those of other terrestrial metallic Ni0 occurrences. Native nickel grains, documented here for the first time in oceanic sediments, appear in calcareous oozes intercalated with metalliferous layers, layers with bedrock rubble, and sporadically scattered cosmic microspherules. There are three possibilities for the metallic Ni0 genesis: (1) cosmic, (2) lower mantle at the core‐mantle boundary, or (3) upper mantle/lower crust. The flake‐like appearance of the Ni0 grains rules out cosmic ablation origin. No unequivocal evidence was found that the Ni0 particles could have formed under high‐pressure conditions similar to those at the core‐mantle boundary. Ni0 formation through seawater serpentinization of the upper mantle/lower crust is the most plausible mechanism. At the slow spreading MAR, seawater penetrates down through highly fractured crest zone and serpentinizes the olivine and pyroxene of the upper mantle/lower crust ultramafics/mafics. Serpentinization results in the generation of highly reduced volatile species: H2 and hydrocarbons. In such an extremely reduced environment, Fe and other transition metals (Co, Ni, Cu, Zn) released from olivines and pyroxenes tend to appear as native metals if fS2 approaches 0. The lightened serpentinized blocks uplift and expose the serpentinized ultramafics/mafics at the rift valley's walls. I speculate that tectonic movements and weathering could liberate the native metallic grains scattered in the serpentinized rocks, which may then have been dispersed in the proximate sediments by near‐bottom currents.

Sorosite (η-Cu 6Sn 5)-bearing native tin and lead assemblage from the Mir zone (Mid-Atlantic Ridge, 26°N)

A number of small, irregular-shaped and spherical shiny metallic particles have been found in the sediments from the Mir zone, Trans-Atlantic Geotraverse hydrothermal field (Mid-Atlantic Ridge, 26°N). The phase variety of the particles examined is represented by metallic tin, tin-rich lead, and tin–copper phases. A detailed mineralogical study of these particles was carried out using optical microscopy, nuclear microscopy, scanning electron microscopy, electron microprobe, proton microprobe and X-ray diffraction analysis. Tin–lead grains have the typical eutectic microtexture of the metal components. Tin–copper grains are formed from single crystals of sorosite, η-Cu 6Sn 5. The Sn–Pb–Cu complex grains are composed of fine stannoan lead (Pb 0.74Sn 0.26) and tin crystals as well as fine (or occasionally larger) sorosite (Cu 6.1Sn 4.9) idiomorphic crystals, in a tin–lead matrix forming eutectic microtexture. On the basis of data obtained, a natural origin is proposed for the examined Sn–Pb–Cu association, and its parent relations with the tectono-magmatic events in the rift zone. This association has probably been formed (1) during the hydrothermal seepings through the Trans-Atlantic Geotraverse sediment cover, or (2) during the evolution of ridge crest magmatic systems. Crystallisation sequence from an initial melt with falling temperature is: firstly sorosite (Cu 6Sn 5) ( T ≤ 380°C) → crystallisation of tin crystals ( T ≤ 227°C) → crystallisation of Sn–Pb eutectic mixture, composed of tin + stannoan lead ( T ≤ 183°C) → limited tin exsolutions in stannoan lead ( T << 183°C). Sn–Pb–Cu grains might be present as accessory minerals in the basic rocks of the east rift wall, which have undergone degradation, permitting their deposition into rift valley sediments.

Native copper and α-copper–zinc in sediments from the TAG hydrothermal field (Mid-Atlantic Ridge, 26°N): nature and origin

Native copper and α-copper–zinc occur as strands and elongated grains (up to 300 μm in length) within the sediments from the Trans-Atlantic Geotraverse (TAG) hydrothermal field (Mid-Atlantic Ridge, 26°N). They are remarkably similar in composition and crystal structure to copper and copper–zinc occurrences found in other natural environments. The results of mineralogical studies are discussed in terms of the possible mechanisms of native metal formation in the complex TAG field: with an asymmetric and highly-fractured rift valley, and mature active and relict sulfide mounds. Native copper and α-copper–zinc grains disseminated in the TAG sediments are either inherited from: (1) primary magmatic or metamorphic crustal source; (2) hydrothermal deposits; and (3) the alteration of primary deposits, or formed (1) authigenically, or (2) biogenically within/on the sediment cover. Native metallic particles could have been formed as accessory minerals disseminated in the ridge crest basic rocks and/or massive sulfide mounds. Degradation of these rocks and mass wasting of the mounds have liberated the metallic grains which have, in turn, been deposited into adjacent sediments.

New Observations on the Distribution of Past and Present Hydrothermal Activity in the TAG Area of the Mid-Atlantic Ridge (26°08′ N)

Seafloor acoustic and photographic imagery combined with high- resolution bathymetry are used to investigate the geologic and tectonic relations between active and relict zones of hydrothermal venting in the TAG (Trans-Atlantic Geotraverse) hydrothermal field at 26°08′N on the Mid-Atlantic Ridge (MAR). The TAG field consists of a large, currently active, high-temperature mound, two relict zones (the Alvin and Mir zones), and an active low-temperature zone. The active mound and the Alvin relict zone lie along a series of closely-spaced, axis-parallel (NNE-trending) faults in an area of active extension east of the neovolcanic zone. The Alvin zone extends for 2.5 km along these faults from the valley floor onto the eastern wall, and consists of at least five mounds identified using DSL-120 sidescan sonar and bathymetric data. The existence of sulfide structures on most of these mounds is verified with near-bottom electronic still camera (ESC) images from the Argo-II deep-towed vehicle, and is confirmed in at least one case with collected samples. Two of these mounds were previously unidentified. The existence of these mounds extends the length of the Alvin zone by ~0.5 km to the south. Much of the Alvin relict zone appears to be buried by debris from a large mass wasting event on the eastern wall of the median valley. The Mir zone, located on normal fault blocks of the eastern valley wall, cannot be clearly identified in the sidescan data and no structural connections from it to the active mound or Alvin zone can be discerned. The active mound is located at the intersection of an older oblique fault set with the younger axis- parallel faults which extend into the Alvin relict zone, and no fresh volcanics are observed in the vicinity of the mound. The fact that both the active mound and the Alvin relict zone lie along the same set of active, axis-parallel faults suggests that the faults may be a major control on the location of hydrothermal activity by providing pathways for fluid flow from a heat source at the ridge axis.

Reduced crustal magnetization beneath the active sulfide mound, TAG hydrothermal field, Mid-Atlantic Ridge at 26°N

A detailed near-bottom magnetic field survey was carried out by the submersible Alvin over the actively venting mound located within the TAG (Trans-Atlantic Geotraverse) hydrothermal field on the Mid-Atlantic Ridge at 26°08′N, 44°49′W. Three-dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound. This magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure. In contrast, the sea surface magnetic anomaly is associated with a broad 2 × 8 km magnetization low elongated along the axis, that includes both the active and inactive mounds. The short-wavelength (250 m), near-bottom magnetic anomaly over the active mound is far too small to produce the long-wavelength (8 km) sea surface magnetic anomaly at TAG however, and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly. Other hypotheses, including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely. The existence of vigorous hydrothermal activity at TAG and the lack of microseismic activity in the TAG area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly. The thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long-wavelength sea surface anomaly. Pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long-wavelength magnetic signal could be preserved in the crust. The overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites. These studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late-stage intrusive bodies [1]. While detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones, only drilling will ultimately test these hypotheses.

Black smokers on the Mid‐Atlantic Ridge

Evidence for a variety of active hydrothermal venting phenomena, including black smokers, was discovered at a site on the Mid‐Atlantic Ridge by a research team of government and university scientists. The work was accomplished on a cruise of the National Oceanic and Atmospheric Administration (NOAA) ship Researcher during July 1985 as part of the NOAA Vents Program. The site of the venting phenomena is the Trans‐Atlantic Geotraverse (TAG) Hydrothermal Field on the east wall of the rift valley of the Mid‐Atlantic Ridge at 26°N. The TAG Hydrothermal Field was the first hydrothermal field to be found on an oceanic ridge; it was discovered 12 years ago in 1973. However, until the present cruise, only low‐temperature hydrothermal activity had been documented, and the existence of higher‐temperature hydrothermal activity along slow spreading oceanic ridges (half rate 2 cm/yr), such as the Mid‐Atlantic Ridge, was in question.

TAG Hydrothermal Field: Mid-Atlantic Ridge crest at latitude 26°N

Results of ongoing interdisciplinary investigation of the TAG Hydrothermal Field since discovery in 1972 by the NOAA Trans-Atlantic Geotraverse (TAG) Project are synthesized to document the first and only known active submarine hydrothermal field on a slow-spreading oceanic ridge. The structural setting of the field is the E wall of the rift valley of the Mid-Atlantic Ridge at lat. 26°N where high-intensity hydrothermal activity is favoured by exceptionally close spacing of faults (tens of metres) that enhances permeability and facilitates voluminous circulation, and by projection of the wall directly over intrusive heat sources beneath the rift valley that increases thermal gradients which vigorously drive the upwelling limb of a sub-seafloor hydrothermal convection cell discharging through faults in the wall. A low in residual magnetic intensity coincides with the field and is attributed to hydrothermal alteration of the magnetic mineral component of basalt in the discharge zone. Hydrothermal precipitates in the form of manganese oxide crusts of extreme purity (40% Mn) and rapid deposition rate (200 mm 10-6 yr) cover about 10% of the sea-floor within the field, distributed along faults sub-parallel to the rift valley. The composition and mineralogy of basalts exposed at the sea-floor is indicative of low-temperature zeolite metamorphic facies. Thin sediments of the field exhibit average metal-to-aluminum ratios and non-detrital metal accumulation rates (Fe, Mn, Ni, Co, Cu, Cr) that are high relative to other areas of the Mid-Atlantic Ridge. Present hydrothermal activity is evidenced in the near-bottom water by temperature anomalies and excesses of the primordial isotope 3He, and in the surrounding water column by increases in weak-acid soluble suspended particulate matter enriched in certain metals (Fe × 10; Mn 10%). The observed distribution of hydrothermal products indicates that the special structural and thermal conditions that have concentrated hydrothermal activity in the TAG Hydrothermal Field have persisted for at least 1.4 × 106yr.